1. Field of the Invention
The present invention relates to a two-shaft continuous mixer, and more particularly to a two-shaft continuous mixer used for homogenization of plastics, master batch, manufacture of compounds of composite plastics, and the like.
The present invention further provides a continuous mixer in which a plastic material such as various plastics, rubber, etc., is mixed and made molten into a flowing state, which material is supplied to granulation equipment or the like. In particular, in an apparatus of the type in which a gear pump as a material discharge means is directly connected to a material discharge opening, the mixing degree is positively controlled within the mixer under the constant feed condition of the gear pump to thereby perform proper and good mixing over the wide range of viscosity under the stabilized operation, thus providing enhancement and stability of quality.
2. Description of the Prior Art
As prior art there have been previously disclosed U.S. Pat. Nos. 315,408, 3,704,866, 4,310,251 and 4,452,750.
Recently, a continuous mixer with a gear pump (with closed coupling) has been used, which is of the energy saving type, in place of a continuous mixer with an extruder (with open chute). However, in a system which makes use of pressure control of the continuous mixer with a gear pump, it is difficult to control resins from a low viscosity to a high viscosity. In order to make the wide range of resins applicable, the present inventors have proposed (1) a change in shape of a rotor itself and (2) an addition of the control means for pressure control of the continuous mixer with a gear pump. Of these, the change in shape of the rotor is the technique which can be also applied not only to the continuous mixer with a gear pump but to the continuous mixer with an extruder.
There is a conventional two-shaft continuous mixer which comprises on one side in an axial direction a material supply portion adapted to receive a measured material, and on the other end in an axial direction two rotors having a feed screw portion and a mixing blade portion, said rotors being disposed within a mixing portion extending in a horizontal direction and having a discharge portion, said rotors being arranged in parallel but which are not meshed with each other and are rotatable in different directions.
In this mixer, material in the screw portion of the rotor is continuously fed to the mixing portion, where the material is well mixed through the shearing action between the rotor and the inner surface of the mixing portion (chamber), the force-back action obtained by the special shape of rotors, kneading by mutual action of two rotors, and scattering action, the mixed material is fed to the discharge portion by the pumping action in the rotor discharge portion, and the quality of mixing is controlled by adjusting the degree of opening of an orifice gate.
The rotor in the mixer constructed as described above is designed as shown in FIGS. 5 and 6.
A rotor 1 comprises a feed screw portion 2 and a mixing blade portion 5 which comprises a helical feed blade 3 and a helical return blade 4, said feed screw portion 2 and said mixing blade portion 5 being disposed in an axial direction.
The two rotors 1 are disposed parallel to each other and are rotatable in different directions within the mixing portion 6 substantially in the form of glasses.
The feed blade 3 and the return blade 4 are respectively of the double type and their helical configuration is continuous in an axial direction. In each rotor 1, an intersection 7 between the terminating end of the feed blade 3 and the beginning end of the return blade 4 is axially deviated and an intersection 7 between the two rotors 1 is also deviated. Axial length L.sub.1 from the discharge end of the rotor to a portion, being the average value of the two intersections 7, is 1.66 D with respect to the inside diameter D.sub.1 of the mixing portion 6. The two rotors 1 are asymmetrical with respect to each other, and the number of revolutions thereof is in the speed ratio of approximately 1 : 1.14.
A continuous mixer for various rubber and plastic materials has been known which is of the type wherein a gear pump is directly connected to a discharge opening for the already mixed material for discharging and transporting the material and to favorably influence the mixed material by changing the rotational speed. Prior art of this kind include Japanese Patent Publication No. 3091/81, though not shown, which discloses that a gear pump driven by a variable speed motor is directly connected to a material discharge opening in the mixer to thereby vary the rotational speed of the gear pump and vary inlet pressure of a pump (or an extruder) whereby a filling degree of material within the mixer is varied (i.e., a wetted surface is varied) to control the strength of mixing; and Japanese Patent Application Laid-Open No. No. 183210/83, though not shown therein, which discloses that the continuous mixer is directly connected to the gear pump, an immovable fixed dam which is projected into a space between the known mixing rotors provided parallel with each other is provided within a chamber in the vicinity of a discharge opening of the mixer, in an attempt to cope with the case where the stabilized operation is rendered difficult resulting from a great fluctuation of pressure (occurrence of hunting phenomenon), to interrupt an axial flow of the mixed material, thus reducing the pressure hunting and thereby obtaining the stabilized operating condition.
In the two-shaft continuous mixer which uses the rotors 1 shown in FIGS. 5 and 6, since the zone of the shearing action by the mixing blade portion 5 is lengthy, resin the temperature exceeds 250.degree. C. if polyethylene of high density is used. In particular, this phenomenon is significantly affected by the aforementioned L.sub.1, and accordingly, there is posed a problem of using such a material as just mentioned. Furthermore, the speed ratio is different between the rotors 1, and the feed blade 3 and the return blade 4 are continous in the form of an axial helical configuration and are asymmetrical. Therefore, kneaded spots likely occur and good homogenization is not expected.
More specifically, a short path due to the continuous blades occurs, which possibly leads to formation of nonmolten resin when linear polyethylene of low density is used.
In the current situation which involves the wide range of characteristics of plastic such as superhigh viscosity and superlow viscosity, the two-shaft continuous mixer is an exclusive-use machine despite the fact that a multi-function for uniformly mixing various materials is requested, and the mixer merely has a single function, thus failing to enlarge the possible uses thereof.
The above-described two prior art references, Japanese Patent Publication No. 3091/81 and Japanese Patent Application Laid-Open No. 183210/83 may be summarized as follows: In Japanese Patent Publication No. 3091/81, theoretically a good result should be obtained by the system disclosed therein but actually plastic materials to be handled are multifarious, and the properties thereof are delicately different from one another and the conditions therefor are also different. Therefore, the discharge pressure is materially varied, sometimes failing to achieve the stabilized operation. It can be said that Japanese Patent Application Laid-Open No. 183210/83 has been proposed as an improvement to solve the aforesaid problem. However, only these prior arts are not sufficient to solve the remaining problems which still need be solved regarding the following points. The greater part of the plastic materials used in the continuous mixer comprise plastic materials such as rubber, plastics, etc. These materials, however, have an extremely wide range of viscosity from an extremely low viscosity to an extremely high viscosity. This viscosity is the factor which greatly influences the mixing content and flowing condition of material, which is very difficult to adequately cover all the materials only be varying the rotational speed in the gear pump. Even if an immovable fixed dam is provided within the chamber in addition to the gear pump to interrupt the axial flow of material, there still is posed a problem described in the following discussion: For example, where high viscosity resin material is used, heat generation of the material is great due to the presence of the fixed dam, and therefore it is necessary to minimize the discharge pressure thereof but this provides a variation in engagement of the teeth, resulting in a failure of expecting stabilized operation. In addition, pump surging or the like causes, for example, unevenness of pellets in the granulation equipment to which the material is fed, and mechanical vibrations occur. Where the low viscosity resin material is used, only the heat generation of material due to the presence of the fixed dam is insufficient, and therefore, if the set pressure is increased, the resin material leaks out of a sealed portion, and a feed neck due to a counterflow of the molten resin occurs. These troubles are difficult to be solved when using only the aforesaid prior art references.